Which device controls the duration of time that kVp is applied to the x-ray tube?

The exposure timer is the device responsible for controlling the duration of time that kilovoltage peak (kVp) is applied to the x-ray tube. It regulates how long the x-ray exposure lasts, which is essential for producing images with the appropriate density and contrast. By determining the exposure time, the timer ensures that the x-ray tube operates for the correct duration to generate the desired amount of x-ray photons while preventing overexposure or underexposure of the film or digital detector.

In contrast, the auto transformer is primarily used to supply the proper voltage to the x-ray tube during operation, while the step-up transformer is responsible for increasing the voltage to the required level for x-ray production. The control panel is the user interface that allows the technologist to set various parameters for the exposure, including the kVp and exposure time, but it does not directly manage the timing itself. Thus, the exposure timer is the critical component that directly influences how long the kVp is applied.